With respect to a coil spring (for example a valve spring, suspension spring and the like used for an engine, suspension and the like) used for an automobile and the like, in order to reduce exhaust gas and to improve fuel economy, reduction of the weight has been required and high strengthening has been demanded. Because a high-strengthened spring is liable to cause hydrogen embrittlement, the spring steel wire used for manufacturing the spring is required to be excellent in hydrogen embrittlement resistance.
As a method for improving the hydrogen embrittlement resistance of a high-strength spring steel wire, it is known to control the chemical composition and microstructure and so on. However, according to these methods, a large amount of alloy elements is used which is not necessarily desirable from the viewpoints of the manufacturing cost and resources saving.
Also, as a method for manufacturing a spring, a method of heating a steel wire to a quenching temperature, hot-forming the same into a spring-shape, and thereafter tempering the same by cooling in oil and a method of quenching and tempering a steel wire and thereafter cold-forming into a spring-shape are known. Further, in the cold-forming method of the latter, it is also known to perform the quenching and tempering before forming by high frequency induction heating, and in Patent Literature 1 for example, a technology is known in which high frequency induction heating is performed after cold drawing, and the microstructure is controlled by quenching and tempering. According to the technology, delayed fracture property is improved, and it is achieved by making the microstructure fraction of pearlite 30% or less and the microstructure fraction of martensite or bainite 70% or more, thereafter performing cold drawing with a predetermined area reduction ratio, then performing quenching and tempering, and thereby reducing the amount of undissolved carbide that becomes the origin of the delayed fracture.
According to Patent Literature 2, in the example, a rolled material is drawn and is high frequency heated for quenching and tempering treatment. According to the technology, a spring is achieved which secures high strength and high toughness while securing coiling performance. However, in the technology, the coiling performance is mainly addressed, and no consideration has been made with respect to hydrogen embrittlement resistance.